U.S. patent number 4,368,230 [Application Number 06/127,319] was granted by the patent office on 1983-01-11 for photomask.
This patent grant is currently assigned to VLSI Technology Research Association. Invention is credited to Masatoshi Migitaka, Koichiro Mizukami.
United States Patent |
4,368,230 |
Mizukami , et al. |
January 11, 1983 |
Photomask
Abstract
A photomask comprises a transparent film of conductive material
and a light shielding film of predetermined pattern on a
transparent substrate. The pattern film is made of a metallic
element having its atomic number not smaller than 25 or a
composition containing the metallic element. The photomask
structure is suited to the case where the pattern formed on the
mask is inspected with an electron beam. With the mask structure,
the contrast of a pattern related information signal (backscattered
electrons, secondary electrons, absorption current, etc.) derived
from the mask upon irradiation thereof with the electron beam is
improved.
Inventors: |
Mizukami; Koichiro (Kokubunji,
JP), Migitaka; Masatoshi (Kokubunji, JP) |
Assignee: |
VLSI Technology Research
Association (JP)
|
Family
ID: |
12443234 |
Appl.
No.: |
06/127,319 |
Filed: |
March 5, 1980 |
Foreign Application Priority Data
|
|
|
|
|
Mar 28, 1979 [JP] |
|
|
54-35492 |
|
Current U.S.
Class: |
428/203; 427/109;
427/165; 427/269; 427/74; 428/209; 428/210; 428/432; 428/433;
428/434; 428/457; 428/469 |
Current CPC
Class: |
G03F
1/86 (20130101); Y10T 428/31678 (20150401); Y10T
428/24926 (20150115); Y10T 428/24917 (20150115); Y10T
428/24868 (20150115) |
Current International
Class: |
G03F
1/08 (20060101); B32B 003/00 () |
Field of
Search: |
;428/432,469,210,457,203
;427/109,74 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Lesmes; George F.
Assistant Examiner: Buffalow; E. Rollins
Attorney, Agent or Firm: Antonelli, Terry & Wands
Claims
We claim:
1. A photomask suitable for inspecting a pattern formed thereon
with electron beams comprising:
a transparent substrate comprised of glass;
a transparent electrically conductive film comprised of metal oxide
formed of at least one first metallic element; and
a light shielding film of predetermined pattern provided on said
transparent electrically conductive film, said pattern film being
comprised of at least one material selected from the group
consisting of a second metallic element having an atomic number not
smaller than 25 and a composition containing said second metallic
element, said first and second metallic elements being different
from each other and the ratio of the electron backscattering
coefficient of said pattern film to that of said transparent
electrically conductive film being equal to or larger than about
1.3
2. A photomask suitable for inspecting a pattern formed thereon
with electron beams comprising:
a transparent substrate comprised of glass;
a light shielding film of predetermined pattern provided on said
substrate, said pattern film being comprised of at least one
material selected from the group consisting of a metallic element
having an atomic number not smaller than 25 and a composition
containing said metallic element; and
a transparent electrically conductive film being comprised of metal
oxide formed of at least one metallic element and provided on a
surface of said substrate including said pattern film, the metallic
elements forming said light shielding film and the transparent
electrically conductive film being different from each other and
the ratio of the electron backscattering coefficient of said
pattern film to that of said transparent electrically conductive
film being equal to or larger than about 1.3.
3. A photomask according to claim 1 or 2, wherein the thickness of
said pattern film is equal to or larger than about 0.2 times of the
mean range of electron in the material forming said pattern
film.
4. A photomask according to claim 1 or 2, wherein said metallic
element of said pattern film is one selected from a group
consisting of gold, platinum, tungsten, tantalum, lead and
molybdenum.
5. A photomask according to claim 1 or 2, wherein the thickness of
said transparent conductive film is not larger than about 2,000
A.
6. A photomask according to claim 1 or 2, further comprising an
adhesive film provided between said transparent conductive film and
said pattern film.
7. A photomask suitable for inspecting a pattern formed thereon
with electron beams comprising, on a transparent glass substrate,
an electrically conductive transparent film and a light shielding
film of predetermined pattern, said light shielding film being
formed of at least one metal selected from the group consisting of
gold, platinum, tungsten, tantalum, lead and molybdenum and said
electrically conductive transparent film being formed of metal
oxide of at least one metal selected from the group consisting of
indium, tin, titanium and zinc, the thickness of said light
shielding film of a predetermined pattern being equal to or larger
than about 0.2 times the mean range of electron in the material
forming said pattern film.
8. A photomask according to claim 7, wherein the thickness of said
electrically conductive transparent film is not larger than about
2000 A.
9. A photomask suitable for inspecting a pattern formed thereon
with electron beams comprising:
a transparent glass substrate;
a transparent electrically conductive film formed on said
transparent glass substrate, said transparent electrically
conductive film being formed of metal oxide of at least one
metallic element having an atomic number equal to or smaller than
50; and
a light shielding film of predetermined pattern provided on said
transparent electrically conductive film, said pattern film being
formed of at least one material selected from the group consisting
of another metallic element having an atomic number not smaller
than 72 and a composition containing said another metallic
element.
10. A photomask according to claim 9, wherein said another metallic
element is selected from the group consisting of gold, platinum,
tungsten, tantalum and lead.
11. A photomask according to claim 10, wherein said metallic
element of the conductive film is at least one metal selected from
the group consisting of indium, tin, titanium and zinc.
12. A photomask according to claim 7 or 9, wherein the ratio of the
electron backscattering coefficient of said pattern film to that of
said transparent electrically conductive film is equal to or larger
than about 1.3.
13. A photomask according to claim 9, wherein the thickness of said
light shielding film is in a range of from 1500 to 2000 A.
Description
BACKGROUND OF THE INVENTION
The present invention relates to the improvement on photomasks used
for the fabrication of semiconductor integrated circuits, and more
particularly to the improvement on photomasks suited to the case
where patterns formed thereon are inspected while using electron
beams as probe means.
The inspection of a pattern on a photomask with an electron beam
includes successively irradiating the surface portions of the
photomask under consideration with the electron beam and detecting
a signal representative of pattern related information derived from
the mask. As such a signal can be used backscattered electrons,
secondary electrons, absorption current, etc. The checking of an
object surface condition utilizing backscattered electrons,
secondary electrons, absorption current, etc. derived from the
object upon irradiation thereof with an electron beam is well
known. For example, U.S. Pat. No. 3,381,132 discloses the analysis
of a specimen utilizing secondary electrons and backscattered
electrons, and U.S. Pat. No. 3,549,999 discloses the testing of
integrated circuits utilizing secondary electrons. Since the
intensity of the above-described information signal varies
depending upon the kind of the mask surface material, it is
possible to inspect the configuration and size of the pattern by
detecting such a signal while successively scanning the mask
surface with the electron beam.
The electron beam scanning process would require provision for
preventing the mask surface from being charged by the scanning
electrons. One approach for that purpose is proposed by U.S.
application Ser. No. 925,791 filed July 18, 1978 by Koichiro
Mizukami and Masatoshi Migitaka, now U.S. Pat. No. 4,256,778, and
assigned to the assignee of the present application (corresponding
to German patent application No. P2832151.9 filed July 21, 1978).
This patent application proposes to dispose a transparent
conductive layer on the mask surface or to make the mask substrate
of a transparent conductive material. The electron beam is
irradiated under a condition in which the conductive layer or
substrate is connected to a predetermined potential such as a
ground potential. Various examples of such a conductive mask are
shown in FIGS. 1A to 1C. In the figures, reference numeral 1
designates a transparent substrate of insulating material such as
glass, 2 a transparent film of conductive material, 3 a mask
pattern or light shielding film of chromium (Cr), and 4 a
transparent substrate of conductive material.
FIG. 2A schematically illustrates a signal obtained when the
vicinity of the pattern of the mask shown in FIG. 1A is scanned
with an electron beam. The mask pattern defines that portion of the
mask serving as a light shield when the fingers on the mask is
transferred onto a resist, and is hereinafter referred to simply as
"pattern." An actual signal which can be observed would include
noises as shown in FIG. 2B. The increased beam scanning rate for
reducing a pattern inspection time would require a correspondingly
increased signal detecting speed. In that case, the noises
contained in the detected signal would be enhanced, as shown in
FIG. 2C, thereby rendering the discrimination of a difference in
signal intensity due to the presence/absence of the pattern
difficult and remarkably deteriorating the reliability of results.
However, if the signal intensity is obtained with a greatly
distinguishable difference between pattern and non-pattern
portions, as shown in FIG. 2D, it can be expected that the
presence/absence of the pattern may be correctly recognized even if
large noises are contained in the detected signal. In other words,
if a mask itself is constructed so that it exhibits a high contrast
(the ratio of a pattern related signal intensity to a non-pattern
related signal intensity, and vice versa, namely, the ratio of the
higher intensity to the lower intensity) through the scanning by an
electron beam, the signal detecting speed can be enhanced without
deteriorating the reliability of detected results.
FIG. 3 shows the present inventors' experimental results in which
the contrast of backscattered-electron signal derived upon
irradiation with electron beams is measured for mask samples which
have the same structure as that of FIG. 1A and comprise a film of
indium oxide In.sub.2 O.sub.3 (the actually used material
containing a small amount of Sn and having the In/O ratio slightly
deviated from 2/3) of a thickness of d.sub.1 .apprxeq.2,000 A as
the transparent conductive layer 2 and Cr films of various
thicknesses d.sub.2 as the pattern film 3. It can be seen from FIG.
3 that the sample having the thickness of d.sub.2 =1,000 A exhibits
the maximum contrast. But, the maximum contrast as low as
approximately 1.1 cannot allow an increase of checking speed. A
main factor for the low contrast is considered to be a contribution
of the underlying conductive material to the backscattered-electron
signal intensity resulting from the penetration of an incident
electron beam throughout the Cr film 3. Though the thickness
d.sub.2 of the Cr film increased up to a value at which
backscattered electrons from the underlying material are negligible
would provide a signal intensity indicating a difference in surface
material, it has been found that the thickness d.sub.2 above 5,000
A is necessary under an electron beam accelerating voltage of 20 KV
for that purpose. Further, such an increased thickness d.sub.2 of
pattern film makes the formation of a finely defined pattern
difficult. Though the required pattern thickness can be decreased
if the beam accelerating voltage is lowered, there would be
drawbacks such as the deterioration of S/N ratio due to the
decrease of beam current and the instability of beam position.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a photomask in
which the contrast of pattern related information signal derivable
upon irradiation with an electron beam is high.
A photomask of the present invention comprises on a transparent
substrate a transparent film of conductive material and a light
shielding film of predetermined pattern. According to one aspect of
the present invention, the pattern film is made of a metallic
element having its atomic number not smaller than 25 (preferably,
not smaller than 72) or a composition containing the metallic
element. According to another aspect of the present invention, it
is preferable that the ratio of the electron backscattering
coefficient of the pattern film to that of the transparent
conductive film is equal to or larger than about 1.3 (as the
optimum value, equal to or larger than 1.5). Also, it is preferable
that the thickness of the pattern film is equal to or larger than
about 0.2 times (as the optimum value, 0.5 times) of the mean range
of electron in the material forming the pattern film.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1C show in cross section examples of a photomask
disclosed in the U.S. application Ser. No. 925,791 (or German
patent application No. P2832151.9).
FIGS. 2A to 2C show schematic waveforms of signals derived from the
vicinity of a pattern when the photomask shown in FIG. 1A is
scanned with an electron beam.
FIG. 2D shows a schematic signal waveform for explaining an ideal
contrast of signal intensity between pattern and non-pattern
portions.
FIG. 3 graphically shows a relationship between the pattern film
thickness and the backscattered-electron contrast measured by the
present inventors' experiments for the photomask shown in FIG.
1A.
FIG. 4 graphically shows a relationship between the backscattering
coefficient of a thin film and the thickness of the thin film which
is useful in explaining the principle of the present invention.
FIG. 5 graphically shows a relationship between the backscattering
coefficient for a structure having a thin film formed on a
substrate material, and the thickness of the thin film, which is
useful in explaining the principle of the present invention.
FIG. 6 graphically shows a relationship between the contrast for a
mask having a pattern film formed on a conductive film, and the
thickness of the pattern film, which is useful in explaining the
present invention.
FIG. 7 shows in cross section a main part of a photomask according
to one embodiment of the present invention.
FIG. 8 graphically shows the contrasts obtained when the thickness
of the Au film is changed in the photomask of FIG. 7.
FIGS. 9 and 10 show in cross section the respective main parts of
photomasks according to other embodiments of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now, the principle and various embodiments of the present invention
will be explained referring to FIGS. 4 to 10.
A general relationship between the thickness of a thin film and the
intensity of backscattered electrons derived from the thin film (or
the amount of charges generated as backscattered electrons per unit
time) upon irradiation thereof with an electron beam is illustrated
in FIG. 4. In the figure, the ordinate represents the
backscattered-electron intensity normalized by a beam current (i.e.
a current entering the thin film). The normalized intensity is
equal to the so-called backscattering coefficient .eta. which
defines the total flow or number of backscattered electrons
generated per incident electron. FIG. 4 shows that the
backscattered-electron intensity increases with the increase of
film thickness d but exhibits a constant value inherent to the film
forming material if the thickness d exceeds a half of the main
range R.sub.m of electron in the film forming material. The
magnitude R.sub.m is defined as a penetration depth at which the
average energy of electron entering a material under consideration
becomes zero, the depth depending upon the kind of the used
material (and the magnitude of a beam accelerating voltage).
When a thin film made of a material having its backscattering
coefficient .eta..sub.2 is formed on a material having its
backscattering coefficient .eta..sub.1, a relationship between the
backscattered-electron intensity or backscattering coefficient
.eta. for the composite structure and the thickness d of the thin
film would be assumed as shown in FIG. 5 if .eta..sub.1
<.eta..sub.2. Therefore, for a mask in which a pattern made of a
material having its backscattering coefficient .eta..sub.2 is
formed on a conductive substrate having its backscattering
coefficient .eta..sub.1 (or a glass substrate having a conductive
surface film), one can expect the contrast as shown in FIG. 6. From
the figure, it is understood that the available maximum contrast is
equal to the ratio .eta..sub.2 /.eta..sub.1 and the pattern forming
film thickness d of at least R.sub.m /2 is necessary to obtain the
maximum contrast. From the foregoing, it can be recognized that the
following conditions are advantageous for improving the contrust of
a conductive mask:
(1) that a material having a smaller value of R.sub.m is used as
the pattern forming material; and
(2) that the pattern film forming material and the conductive film
forming material are selected with a large ratio between the
backscattering coefficients .eta. of both materials.
On the other hand, it is known that a material having a smaller
value of R.sub.m generally coincides with one having a larger value
of .eta. and such a material is an element having a larger atomic
number Z or a composition thereof such as a compound or a mixture.
Accordingly, the above conditions (1) and (2) can be represented as
follows:
(3) that it is preferable to use a pattern forming material having
the backscattering coefficient .eta. (or atomic number Z) as large
as possible on one hand and to use a conductive film forming
material having the backscattering coefficient .eta. (or atomic
number Z) as small as possible on the other hand.
When a composition material such as a compound or a mixture is
used, one can take as the atomic number Z the weighted average of
respective atomic numbers of its constituent elements. For example,
the weighted average for a composition A.sub.x B.sub.y including an
element A of atomic number Z.sub.A and an element B of atomic
number Z.sub.B is defined as (xZ.sub.A +yZ.sub.B)/(x+y).
Glass normally used as the mask substrate is one material having a
smaller value of .eta.. Therefore, even if a conductive film formed
on the glass substrate is made of a material having a larger value
of .eta., the intensity of backscattered electrons from the
conductive film gets near that from the glass substrate when the
conductive film is made thinner. Thus, it is possible to
effectively make the backscattering coefficient .eta. of the
conductive film small. The .eta. of the conductive film in that
case can be called "effective" backscattering coefficient and it
should be noted that the term "backscattering coefficient" used in
the present invention with respect to the conductive film includes
the effective backscattering coefficient.
The requirement for only the above condition (2) may be satisfied
by the combination of a conductive substrate having a larger value
of .eta. and a pattern film having a smaller value of .eta.. In the
case, however, a sufficient contrast cannot be attained unless the
pattern film is made thicker. The increased thickness of the
pattern film renders the formation of a finely defined pattern
difficult.
The foregoing explanations have been made in conjunction with the
mask structure shown in FIG. 1A. The same holds for the mask
structures shown in FIGS. 1B and 1C. Also, the similar can be said
when secondary electrons or absorption current are detected instead
of backscattered electrons. Since secondary electrons are produced
by not only primary electrons incident on a sample surface but also
backscattered electrons passing near the surface, the coefficient
.delta. of generation of secondary electrons (or the number of
secondary electrons generated per incident electron) can be
expressed as follows:
Here, .delta..sub.SE is the number of secondary electrons produced
directly by one incident electron and .delta..sub.RE is the number
of secondary electrons produced by one backscattered electron. The
equation (1) shows that .delta. becomes larger with the increase of
.eta. and hence a mask exhibiting a high backscattered-electron
contrast, i.e. a mask having a large ratio between the
backscattering coefficients .eta. of the pattern and non-pattern
portions is also advantageous to the provision of a large ratio
between the secondary-electron generation coefficients .delta..
Assuming that a current entering a sample is I.sub.O, the
absorption current I.sub.AE can be represented as follows:
From the equation (2), it can be understood that the combination of
materials having greatly different backscattering coefficients
.eta. provides a large difference between the absorption currents
I.sub.AE. Therefore, a mask exhibiting a high
backscattered-electron contrast is also advantageous to the
provision of a high contrast of absorption current.
For a photomask of the present invention made on the basis of the
above principle, it has been found that a metallic element having
its atomic number Z not smaller than 72, e.g. a heavy metal such as
tungsten (Z=74), gold (Z=79) and platinum (Z=78), or a composition
containing the metallic element may be preferably used as a pattern
forming material. Also, it has been found that the ratio of the
backscattering coefficient .eta..sub.2 of the pattern film to that
.eta..sub.1 of the transparent conductive film is preferably
selected to be equal to or larger than about 1.3 (as the optimum
value, equal to or larger than 1.5) and the thickness of the
pattern film is preferably selected to be equal to or larger than
about 0.2 times (as the optimum value, 0.5 times) of the mean range
R.sub.m of electron in the pattern film forming material.
FIG. 7 shows a photomask according to one embodiment of the present
invention. In the figure, reference numeral 1 designates a
transparent substrate of glass, 2 a transparent conductive film of
indium oxide (In.sub.2 O.sub.3) formed on the entire upper surface
of the substrate 1 having a thickness d.sub.1, and 3' a gold (Au)
film of predetermined pattern formed on the transparent conductive
film 2 having a thickness d.sub.2. Though only one Au film is shown
in FIG. 7 for convenience of illustration, the Au film includes a
continuous film having a complicated pattern or a plurality of
separate films. A thin chromium (Cr) or titanium (Ti) film 5
between the Au film 3' and the transparent conductive film 2 having
a thickness of about 100 A in this example is provided for
enhancing an adhesion between the Au film 3' and the In.sub.2
O.sub.3 film 2.
The contrasts measured when the thickness d.sub.2 of the Au film 3'
is changed taking the thickness d.sub.1 of the In.sub.2 O.sub.3
film 2 as a parameter in the photomask shown in FIG. 7, are shown
in FIG. 8. Referring to FIG. 8, curve A is plotted for the case of
d.sub.1 =2,000 A and curve B for the case of d.sub.1
.apprxeq.100-200 A. It should be understood that the contrast of
the photomask for a given value of the pattern or Au film thickness
d.sub.2 can be controlled by changing the thickness d.sub.1 of the
transparent conductive film 2. For example, when the Au film 3' has
a thickness of d.sub.2 =1,000 A, the contrast is about 1.5 in the
case of d.sub.1 =2,000 A and about 3.3 in the case of d.sub.1
.apprxeq.100-200 A and hence a contrast within a range from 1.5 to
3.3 can be obtained by changing d.sub.1 within a range between
2,000 A and 100 A. Further, it can be understood that the smaller
the thickness d.sub.1, the higher the contrast becomes and that the
contrast obtained under a condition of d.sub.1 .ltoreq.2,000 A is
remarkably improved over all the thicknesses of the Au film 3' when
compared with the case of FIG. 3 using the Cr pattern film. Also,
it can be said that a relatively low value 1.3 of the contrast is
sufficiently practical in comparison with FIG. 3. This contrast
value of 1.3 is the maximum contrast available when .eta..sub.2
/.eta..sub.1 =1.3 (.eta..sub.1 : the backscattering coefficient of
the transparent conductive film 2, .eta..sub.2 : the backscattering
coefficient of the Au film 3') is satisfied. Accordingly, it can be
understood that .eta..sub.2 /.eta..sub.1 .gtoreq.1.3 (preferably,
.gtoreq.1.5) provides a sufficient contrast in a practical use.
FIG. 8 further shows that the contrast value saturates in the Au
film thickness.gtoreq.1,500-2,000 A and hence the Au film thickness
of 1,500-2,000 A is enough to obtain the maximum contrast. Thus,
the maximum contrast can be achieved with a relatively small film
thickness. As understood from the explanation referring to FIG. 6,
the thickness providing the maximum or saturated contrast is equal
to or larger than about 0.5 times of the mean range R.sub.m of
electron in the Au material forming the pattern film 3'.
Considering that even in the case where the thickness of the Au
film 3' has a value smaller than the abovespecified value (for
example, 1,000 A corresponding to about 0.2 times of R.sub.m) a
contrast sufficiently higher than the case of FIG. 3 using the Cr
film can be obtained, a practically acceptable thickness of the Au
film 3' is equal to or larger than about 0.2 R.sub.m and the
optimum thickness is about 0.5 R.sub.m.
Though FIG. 8 shows the measured results when the pattern film was
made of gold Au having its atomic number of Z=79, similar results
has been obtained in the cases where the pattern films were made of
tungsten W (Z=74) and platinum Pt (Z=78) respectively.
The review of various materials has revealed that at least one
selected from a metallic element having its atomic number not
smaller than 72 or a composition containing the metallic element
provides preferable results. A suitable metallic element other than
Au, W and Pt may include tantalum Ta (Z=73) and lead Pb (Z=82).
Further, it has been found that a metallic element having its
atomic number larger than that of chromium Cr (Z=24), i.e. the
atomic number not smaller than 25, for example, molybdenum Mo
(Z=42) can be also used.
The transparent conductive film material may be indium oxide or
NESA glass. The use of an oxide of a metallic element having the
atomic number smaller than indium In (Z=49), tin Sn (Z=50), for
example, titanium Ti (Z=22), zinc Zn (Z=30) can provide a higher
contrast.
In the foregoing, the embodiment has been described in conjunction
with the photomask structure shown in FIG. 1A. The present
invention is also applicable to the photomask structures shown in
FIGS. 1B and 1C. FIG. 9 shows a photomask according to another
embodiment of the present invention in which a mask pattern 3' of a
heavy metal such as Au is formed on a transparent substrate 1 of
insulating material such as glass and a transparent conductive film
2 is formed on the resultant structure. FIG. 10 shows a photomask
according to still another embodiment of the present invention in
which a mask pattern 3' of a heavy metal such as Au is formed on a
transparent substrate 4 made of a conductive material. In FIGS. 9
and 10, reference numeral 5 designates a film for enhancing an
adhesion between the pattern film 3' and the conductive film 2 or
the conductive substrate 4, like the film 5 shown in FIG. 7.
The present invention is by no means limited to the above specified
embodiments or descriptions, but admits various modifications or
changes within the spirit of the present invention.
* * * * *